A marine outboard engine generally comprises a bracket assembly that connects the drive unit of the marine outboard engine to the transom of a boat. The drive unit includes the internal combustion engine and propeller. The marine outboard engine is typically designed so that the steering angle and the tilt/trim angles of the drive unit relative to the boat can be adjusted and modified as desired. The bracket assembly typically includes a swivel bracket carrying the drive unit for pivotal movement about a steering axis and a stern bracket supporting the swivel bracket and the drive unit for pivotal movement about a tilt axis extending generally horizontally. The stern bracket is connected to the transom of the boat.
Some marine outboard engines are provided with a hydraulic linear actuator connected between the stern and swivel brackets for pivoting the swivel bracket to lift the lower portion of the outboard engine above the water level or, conversely, lower the lower portion of the outboard engine below the water level. Some marine outboard engines are also provided with a distinct hydraulic linear actuator for pivoting the swivel bracket through a smaller range of angles and at slower rate of motion to trim the outboard engine while the lower portion thereof is being submerged. Some marine outboard engines are also provided with a hydraulic linear actuator connected between the swivel bracket and the drive unit for pivoting the drive unit about the steering axis in order to steer the boat.
In order to operate the one or more hydraulic actuators, hydraulic fluid needs to be supplied to the actuators. This requires one or more pumps, hydraulic fluid reservoirs, and multiple valves and hoses.
Hydraulic actuators such as the hydraulic linear actuators described above are referred to herein as “unbalanced” actuators. A linear actuator has an actuator body inside of which a piston is slidably disposed. A piston rod extends from one side of the piston and passes through an end of the actuator body. Pumping hydraulic fluid on the side of the piston that does not have the rod causes the piston and the piston rod to move such that the rod extends further out of the actuator body. Pumping hydraulic fluid on the side of the piston having the rod causes the piston and the piston rod to move in the opposite direction such that the rod is retracted inside the actuator body. Due to the presence of the piston rod on only one side of the piston, less hydraulic fluid is required to retract the piston rod by one unit of length than to extend the piston rod by one unit of length. Hence the name unbalanced actuator.
Therefore, when pumping fluid in the actuator body on the side of the piston that does not have the piston rod, the volume of hydraulic fluid being pushed out of the actuator body on the other side of the piston is not sufficient to replace the volume of hydraulic fluid being pumped into the actuator body and the pump has to draw additional hydraulic fluid from the hydraulic fluid reservoir to make up the difference. Conversely, when pumping fluid in the actuator body on the side of the piston that has the piston rod, the volume of hydraulic fluid being pushed out of the actuator body on the other side of the piston is greater than the volume of hydraulic fluid needed to push the piston and the excess hydraulic fluid is returned to the fluid reservoir.
This exchange of hydraulic fluid with the reservoir has the advantage of purging air that might be trapped in the hydraulic fluid as it is circulated to and from the actuator. Air trapped in the hydraulic fluid is separated from the hydraulic fluid as it sits in the fluid reservoir and hydraulic fluid that is drawn from the fluid reservoir is substantially free of trapped air.
Other hydraulic actuators, such as some rotary actuators and linear actuators having pistons rods of equal diameters on each side of the piston, are referred to herein as “balanced” actuators because the same or substantially the same amount of hydraulic fluid is required to move them by one dimensional unit in one direction or the other. As such the pump takes hydraulic fluid from one side of the actuator and returns it to the other side of the actuator. In other words, the volumetric flow rate of hydraulic fluid into the actuator is the same as the volumetric flow rate of hydraulic fluid out of the actuator. Therefore, there is no or very little exchange of hydraulic fluid with the fluid reservoir and balanced actuators do not benefit from the air purging feature that inherently occurs with unbalanced actuator.
Therefore there is a need for a hydraulic system for a marine engine having a balanced actuator that provides air purging of the hydraulic fluid.